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Bureau of Mines Information Circular/1985 




An Overview of Research 
on Explosionproof Enclosures 

By L. W. Scott and MR. Yenchek 




UNITED STATES DEPARTMENT OF THE INTERIOR 



T5 



^INES 75TH AV^ 



jw^^>2) 



Information Circular 9051 

An Overview of Research 
on Explosionproof Enclosures 

By L. W. Scott and MR. Yenchek 




UNITED STATES DEPARTMENT OF THE INTERIOR 

Donald Paul Hodel, Secretary 

BUREAU OF MINES 
Robert C. Horton, Director 






Library of Congress Cataloging in Publication Data: 



Scott, Lawrence W 

An overview of research on explosionproof enclosures. 

(Information circular ; 9051) 

Bibliography: p. 17-18. 

Supt. of Docs, no.: I 28.27: 9051. 

1. Mine explosions— Safety measures. 2. Coal mines and mining- 
Safety measures. I. Yenchek, M. R. (Michael R.). II. Title. III. Se- 
ries: Information circular (United States. Bureau of Mines) ; 9051. 



TN295.U4 [TN313] 622s [622'. 334] 85-600164 



^ CONTENTS 

\y~ Page 

Abstract 

Introduction 

Electrical clearances 

Reliability of enclosures with windows 

Glass 

Plastic 

Safety factors in X-P enclosures 

Potting materials used in X-P enclosures 



^ 



^ 






1 


2 


2 


3 


4 


4 


4 


6 


6 


6 


12 


13 


14 


14 


16 


17 


17 



Performance tests for X-P enclosures 

Structural performance 

Ruggedness 

Effects of high-voltage on explosion-protection techniques 
Innovative X-P devices 

Pressure vent 

Elastomeric grommet cable entry 

Summary 

References 

ILLUSTRATIONS 

1. Minimum arc voltage versus air-gap spacings of electrodes 3 

2. Hydrostatic test apparatus 10 

3. Recommended sealant pattern 11 

4. Kinetic energy produced by roof falls at top of continuous-mining machine.. 12 

5. Pressure vent 14 

6. Pressure buildup in vented enclosures 16 

7. Suggested guidelines for number of screens and allowable flange gaps for 

vents and explosionproof electrical enclosures 16 

8. Trailing-cable entry assembly 17 

TABLES 

1 . Summary of safety factors for four enclosures 5 

2 . Arc-decomposition products 7 





UNIT OF MEASURE 


ABBREVIATIONS USED 


IN THIS REPORT 


°c 


degree Celsius 






J/c 


joule per cycle 


°F 


degree Fahrenheit 






kV 


kilovolt 


ft 


foot 






KE 


kinetic energy 


ft 3 


cubic foot 






lb 


pound 


ft-lb 


foot pound 






lb 2 


square pound 


ft-lb/ft 2 


foot pound per square 


foot 


mm 


millimeter 


gal 


gallon 






mph 


mile per hour 


in 


inch 






mV 


millivolt 


in 2 


square inch 






psig 


pound per square inch, gauge 


in 3 


cubic inch 






V 


volt 


in/ft 


inch per foot 






V ac 


volt, alternating current 


in 2 /ft 3 


square inch per cubic 


foot 


V dc 


volt, direct current 


J 


joule 











AN OVERVIEW OF RESEARCH ON EXPLOSIONPROOF ENCLOSURES 

By L. W. Scott ' and M. R. Yenchek 1 



ABSTRACT 

This report presents an overview of explosionproof (X-P) enclosure 
research being conducted by the Bureau of Mines. This report empha- 
sizes the increasing importance of research related to the safety con- 
siderations of X-P enclosures used in underground coal mines. Selected 
topics are included that summarize results of research in the areas of 
electrical clearances, reliability of enclosures with windows, safety 
factors in X-P enclosures, potting materials, performance tests for X-P 
enclosures, protection against high-voltage explosions, and innovative 
X-P devices. 



Electrical engineer, Pittsburgh Research Center, Bureau of Mines, Pittsburgh, PA, 



INTRODUCTION 



For several years, the Bureau of Mines, 
through in-house and contract research, 
analyzed several of the critical factors 
involved in the design of X-P enclosures 
(_1_).2 The study of X-P enclosures is an 
area requiring investigation regarding 
materials, methods of construction, and 
acceptance and testing criteria. The X-P 
enclosures used in underground mines are 
constructed according to rigid design re- 
quirements that contribute to the dif- 
ficulties in maintaining the enclosures 
in a permissible condition. If designers 
were permitted more freedom, it would be 
possible to construct enclosures that are 



easier to maintain in a permissible con- 
dition. These new enclosures would be 
performance tested to ensure that they 
would be as safe as the enclosures being 
constructed to the design requirements. 

The objective of this research is to 
determine the safety factors in the pres- 
ent design requirements and to develop 
and demonstrate the feasibility of per- 
formance tests. The progress presented 
here should be of considerable inter- 
est to the mining industry in general 
and to designers of X-P enclosures in 
particular. 






ELECTRICAL CLEARANCES 



It has been known for many years that 
the flame front of a methane-air explo- 
sion produces free ions that can cause a 
current to flow when the front bridges 
two conductors of opposite polarity. If 
the voltage is high enough, this current 
can increase to a value that would be 
self-sustaining, and a dangerous arc dis- 
charge can occur. Arc discharges pro- 
duced in this way could cause premature 
failure of electrical equipment, produce 
excessive heating, or burn through an en- 
closure wall. This study was initiated 
to obtain experimental results that would 
be useful in determining safe levels of 
voltages and clearances required to pro- 
tect X-P enclosures from failures due to 
arc discharges. 

The critical arcing voltages (single 
phase, alternating current) for vari- 
ous air gaps are shown in figure 1 (2). 
Curve A shows the minimum arcing voltage 
versus electrode spacing in air. This 
curve was obtained by selecting the spac- 
ing and increasing the voltage until arc- 
ing occurred. Curve B shows the minimum 
arcing voltage versus electrode spacing 
in a 9.8% methane-air mixture. Because 
it was necessary that an explosion occur 
for each measurement, it was not possi- 
ble to conduct the experiments as simply 

5 

^Underlined numbers in parentheses re- 
fer to items in the list of references at 
the end of this report. 



as those on arcing-in-air. For curve B, 
again a spacing was selected and the 
voltage was varied until arcing occurred. 
However, at this point, the voltage was 
decreased by 100 V, and the explosion 
test repeated. If arcing occurred, the 
voltage was decreased again by 100 V. 
This procedure was continued until no 
arcing was observed. At the level of 
voltage where no arcing occurred, 10 
tests were conducted. If no arcing was 
observed, the previous level of voltage 
was considered to be the minimum arcing 
voltage. 

The minimum arcing voltage was found to 
increase with the distance between the 
electrodes. It follows that the danger 
of arcing can be decreased in X-P enclo- 
sures by using larger air-gap spacing be- 
tween live conductors of opposite polar- 
ity and between live conductors and 
earth. 

Figure 1 is a plot of the actual mini- 
mum arcing voltage versus air-gap spac- 
ing. However, since these experiments 
were conducted for design purposes, and 
because of limited space for X-P enclo- 
sures, it was decided that a factor of 
1.5 should be applied to the spacing ob- 
tained in figure 1, in order to provide 
a reasonable margin of safety. For exam- 
ple, curve B indicates that arcing will 
occur at a spacing of 1.2 in (30 mm) if 
5,000 V are impressed across the elec- 
trodes in a 9.8% methane-air mixture 



ID 
g 

> 

uj 

o 
> 

- 
_l 
CL 
Q. 
< 



Curve B 




: 



10 



20 



30 



80 



90 



40 50 60 70 

SPACING, mm 

FIGURE 1. - Minimum arc voltage versus air-gap spacings of electrodes. 



100 



no 



(compared to a spacing of 0.071 in (1.38 
mm) for normal atmosphere, as shown in 
curve A). Therefore, a spacing of 1.8 in 
(45 mm) [0.106 in (2.07 mm) for curve A] 
should provide adequate clearance for 
this voltage. 

All clearance data were obtained using 
0.5-in (12.5-mm) diameter spherical brass 



electrodes at room temperature and atmos- 
pheric pressure, thus giving optimis- 
tic values of clearances. The required 
clearance for pointed electrodes are 
considerably greater. Pointed termina- 
tions should be avoided, and every effort 
should be made to employ rounded termina- 
tions with smooth surfaces. 



RELIABILITY OF ENCLOSURES WITH WINDOWS 



Windows in X-P enclosures require care- 
ful design, fabrication, and installa- 
tion. The designer must be certain that 
the window or lens is adequate for the 
design conditions of the enclosure, par- 
ticularly the dynamic pressure, tem- 
perature, point impact, thermal shock, 
and corrosive effect of the underground 
environment. 



Windows and lenses should be fabricated 
only from materials suited for the opera- 
tional environment encountered in mines. 
The suitability of a material is based 
either on documented extensive past ex- 
perience or on exhaustive evaluation by a 
materials testing laboratory in simulated 
mine environments. At present, glass and 
polycarbonate plastic are considered the 



only practical materials for fabrication 
of windows and lenses (3). 

Glass 



The chemical composition, casting pro- 
cess, and thermal treatment determine the 
physical, chemical, optical, and electri- 
cal properties of glass. Because of a 
very complex relationship among these 
variables, no single glass composition, 
casting process, or thermal treatment is 
considered superior to others. Thus, the 
designer is free to select the combi- 
nation of fabrication parameters that 
best matches a specific set of product 
requirements. 

The primary advantages of glass are its 
ability to retain its physical and opti- 
cal properties under high ambient temper- 
ature, ultraviolet radiation, and humid- 
ity for a long period of time; to resist 
surface abrasion by rock particles; and 
to tolerate immersion in aqueous and 
organic solvents without initiation of 
stress cracking or corrosion. Glass win- 
dows can tolerate 100% relative humidity, 
temperature of 400° F (204° C), intensive 
ultraviolet radiation, and continuous or 
intermittent immersion in basic or acidic 
water or organic solvents for indefinite 
periods. 

The primary shortcomings of glass are 
its brittleness and low tensile strength. 
To compensate for these shortcomings, the 
design of the window seat assembly must 
provide, whenever feasible, protection 
against point contact with the metallic 
components of the enclosure and impact by 
rock fragments capable of fracturing the 
window. The protection against fracture 
initiated by point contact is usually ac- 
complished by inserting gaskets between 
the glass and metallic components of the 



seat assembly. Protection against break- 
age by impact is generally provided by an 
external shield in the form of a cage or 
plastic envelope, or by precompressing 
the glass window surfaces with thermal 
tempering or chemical ion exchange. 

Because of their history of success- 
ful use in enclosures, the follow- 
ing are practical for use as windows 
in enclosures: (1) borosilicate glasses, 
(2) soda lime glasses, and (3) silica 
glasses. 

Plastic 



The high temperature, humidity, inten- 
sive ultraviolet radiation, and presence 
of vapors from petroleum-base oils tend 
to rapidly degrade the mechanical proper- 
ties of plastic windows and lenses in X-P 
enclosures. Some plastics deteriorate in 
lamp enclosure service faster than oth- 
ers, but even the most resistant ones age 
sufficiently to mandate their removal 
from service in less than 10 years. For 
this reason, a thorough engineering eval- 
uation of plastic material must be con- 
ducted prior to its selection for ser- 
vice as a window in an X-P enclosure. At 
present, polycarbonate plastic is consid- 
ered practical for fabrication of windows 
and lenses for enclosures; however, the 
Mine Safety and Health Administration 
(MSHA) , U.S. Department of Labor, policy 
limits the design temperature to 240° F 
(115° C). Industrial experience has 
shown that three other restrictions 
should be noted: 

1. Contact with hydraulic oil and pe- 
troleum-based lubricants are prohibited. 

2. Low ultraviolet (UV) environment. 

3. Service life should be limited to 
4 years. 



SAFETY FACTORS IN X-P ENCLOSURES 



The design requirements for X-P enclo- 
sures used in underground coal mines are 
contained in Part 18 of Code of Federal 
Regulations Title 30 (30 CFR 18) ( h) . 
However, these requirements do not allow 
for much deviation and the approval pro- 
cess is based primarily on enclosures 



constructed to these requirements. Any 
innovative design attempts by enclosure 
manufacturers would require substantial 
effort to prove that the enclosure is 
as safe as an enclosure built to the re- 
quirements of 30 CFR 18. 



One phase of Bureau research has con- 
sisted of determinating the charac- 
teristics of materials commonly used 
in enclosures. Such materials include 
steel, aluminum, polycarbonates, and var- 
ious sealants. Emphasis is placed on 
determining (1) how these materials sat- 
isfy design requirements and (2) the min- 
imum safety factors contained in the 
requirements . 

A finite-element computer model was 
used to ascertain the structural integ- 
rity of four existing X-P enclosures (1) . 
A finite-element model consists of many 
small elements, each of which has stress 
and deflection characteristics defined 
by classical theory. By proper element 
selection and by writing any necessary 
constraint equations at the corners or 
"nodes" of the elements, a good estimate 
can be obtained of the stresses generated 



in an enclosure by an internal loading 
function. 

The computer code ANSYS selected to 
perform the finite-element analysis is 
commercially available and contains all 
the capabilities necessary for analyz- 
ing X-P enclosures. It has a library of 
finite elements that includes general 
shells, three-dimensional beams and sol- 
ids , and gap elements . It has the capa- 
bility of performing elastic, elastic- 
plastic, or thermal stress analysis, and 
static and dynamic loading. 

In determining the yielding of the en- 
closures, the von Mises yield criterion 
was used (it assumes that the structure 
will yield when the distortion energy 
equals the distortion energy in simple 
tension). A combined stress, called the 
von Mises stress, is defined as 



VM = 0.707 /(a, - a 2 ) 2 + (o 2 - a 3 ) 2 + (a 3 - a,) 2 ' 



where O), 02, 03 are principal stresses. 
The von Mises criterion expressed in 
terms of stresses states that yielding 
will occur when 



°VM 



where a is the yield stress in simple 
tension. 

In determining the safety factors for 
the four enclosures analyzed, a design 
pressure of 150 psig (as specified by 
MSHA) was used. A safety factor for each 
component of the enclosure was determined 
by 

Safety factor = P V M/Pdesign» 



where P VM is the pressure that will give 
the von Mises stress. 

Safety factors for the four enclosures 
analyzed are summarized in table 1. For 
each enclosure, multiple factors of safe- 
ty were calculated for different compo- 
nents and with different finite-element 
models. Table 1 gives the minimum safety 
factor for each enclosure, which is the 
factor that governs the failure mode of 
the enclosure; other factors were cal- 
culated for evaluation and comparison 
only. 

As table 1 shows , there is a wide vari- 
ation in the strength of X-P enclosures. 
Although enclosure 1 showed a safety 



TABLE 1. - Summary of safety factors for four enclosures 





Enclosure 


Enclosure 


Minimum strength 


Safety 


Enclosure 


description 


volume , 
in 3 


component 


factor 


1 


Steel rectangular box, 
aluminum cover. 


926 




0.73 


2 


Steel rectangular box, 
small steel cover. 


234 




1.60 








3 


Rectangular luminaire, 
aluminum casting. 


140 


Bottom plate, 
aluminum casting. 


1.20 


4 


Cylindrical junction 
box, steel casting. 


142 


Cover, steel casting. 


4.83 



factor of less than 1, all of these en- 
closures have been certified as explo- 
sionproof. However, it should be remem- 
bered that the design criterion of 150 



psig used to compute the safety factors 
is a much more severe load than that pre- 
sented by the internal methane-air explo- 
sion specified by MSHA. 



POTTING MATERIALS USED IN X-P ENCLOSURES 



Potting materials are frequently used 
in X-P enclosures to reduce hazards by 
separating potential ignition sources 
from flammable environments. They may 
serve as heat sinks to quench sparks as 
well as heat from zones where abnormal 
operating conditions arise. Under normal 
conditions, materials serve as an elec- 
trical insulator, whether the material is 
a solid, liquid, or gas. Besides the 
usual mechanical and electrical proper- 
ties exhibited by a potting material, two 
additional factors are important in eval- 
uating materials. One factor concerns 
the flammability of the material. The 
second factor relates to the flammability 
of the products produced when the mate- 
rial is decomposed by an electric arc 
caused by failure of the potted electri- 
cal equipment. 

Organic potting materials will, in gen- 
eral, undergo decomposition, carboniza- 
tion, and burn in an oxidizing atmos- 
phere. In an inert atmosphere, pyrolysis 
is likely to occur. However, no general 
guideline exists for establishing an ac- 
ceptance criterion for potting materials 
for use in X-P enclosures used in under- 
ground mines. 

The purpose of this phase of Bureau 
research was to develop a testing 



methodology where a rank ordering of can- 
didate materials may be established based 
on arc decomposition products. A list of 
the arc decomposition products of seven 
potting compounds evaluated in this study 
is presented in table 2. A prescribed 
number of arcing cycles were produced by 
a cycle timer (cycles per second) that 
when multiplied by the average energy per 
cycle gave the energy dissipated in the 
arc. Results show clearly the potential 
for addition of more sensitive hydrocar- 
bons and hydrogen (_5) . 

Results obtained indicate that a rank- 
ing order could be based on the least 
amount of flammable gas (hydrogen) 
evolved. This is evidenced by results 
obtained during arcing tests conducted 
at 43 J/c for 30 cycles. Therefore, the 
materials tested should be ranked in 
the following order, (from best to 
worst) : 

° Eccosil 3 (solid, liquid, gas). 

° DPR242H (solid). 

° Eccothane (gas). 

o DPR242 (solid). 

o Epoxy (solid). 

° Dow Silicone Oil (liquid). 

o RS#3 (asphaltic solid). 



PERFORMANCE TESTS FOR X-P ENCLOSURES 



Since 1977, the Bureau has conducted 
research to develop performance standards 
for X-P enclosures. These new standards 
are intended to provide a method of ap- 
proving enclosures for use in underground 
mines based solely on the performance of 
the enclosure during specified tests. 
When enclosures are evaluated in this 
way, rather than by specifying certain 
dimensions as in 30 CFR 18, the enclosure 
designers will have more freedom in their 
design approach. 

Two performance tests have been 
developed to date: (1) a structural 



performance (hydrostatic pressure) test 
and (2) a ruggedness test. 

STRUCTURAL PERFORMANCE 

The purpose of the structural perform- 
ance test is to verify that X-P enclo- 
sures are designed for a minimum static 
pressure of 150 psig. 



■^Reference to specific products does 
not imply endorsement by the Bureau of 
Mines . 



TABLE 2. - Arc-decomposition products 



Material 


Environment 


Cycles 


Average energy 
per cycle, J 


Decomposition 


products 


Total combus- 




Component ' 


ppm 


tibles, % 




Nitrogen. . . 


60 


43 


H 2 
CH 4 


22,712 
943 


71.5 




2.9 










C 2 H 2 


5,217 


16.4 










C 2 H 4 


2,609 


8.2 










C 3 H 6 


26 


.1 










C3H4 


48 


.2 










C 6 H 6 


230 


.7 






15 


43 


H 2 
CH 4 
C 2 H 2 
C 2 H 4 


1,458 

87 

109 

16 


87.3 
5.2 
6.5 
1.0 






15 


43 


CH 4 


27 


84.3 






60 


43 


CH 4 


5 


100 






30 


43 


CH 4 


<5 


NC 




Nitrogen. . . 


30 


43 


H 2 


11,288 


94.5 






30 


21.5 


CH 4 


10 


50 




Nitrogen. . . 


30 


21.5 


H 2 


4,101 


91 






60 


43 


H 2 

CH 4 

C 2 H 2 

C 2 H 4 

C 3 H 6 
C3H4 

C 6 H 6 


27,979 

4,496 

4,783 

3,913 

661 

430 

361 


67 
8.4 

11.5 

9.4 

1.6 

1.1 

.9 






15 


43 


H 2 

CH 4 

C 2 H 2 

C 2 H 4 

C 3 H 8 


8,305 
435 
739 
163 

22 


85.9 

4.5 

7.6 

1.7 

.3 






15 


43 


CH 4 
C 2 H 2 


32 
22 


59.3 
40.7 






60 


43 


H 2 
CH 4 
C 2 H 2 
C 2 H 4 


339 

109 

152 

98 


48.6 
15.6 
21.8 
14.0 






30 


43 


CH 4 

C 2 H 2 

C 2 H 4 


<5 
22 
<5 


15.6 
68.8 
15.6 




Nitrogen. . . 


30 


43 


H 2 

CH 4 

2^2 
C 2 H 4 

C 3 H 6 


39,424 

739 

2,935 

1,033 

98 


89.1 

1.7 

6.6 

2.3 

.3 






30 


21.5 


CH 4 

C 2^ 2 

C 2 H 4 


13 
15 
10 


34.2 
39.5 
26.3 




Nitrogen. . . 


30 


21.5 


H 2 

CH 4 

C 2 H 6 
C 2 H 2 
C 2 H 4 


11,864 

526 

24 

926 

441 


86.1 

3.8 

.2 

6.7 

3.2 



See notes at end of table, 



TABLE 2. - Arc-decomposition products — Continued 



Material 


Environment 


Cycles 


Average energy 
per cycle, J 


Decomposition 


products 


Total combus- 




Component 1 


ppm 


tibles, % 


Silicone oil. . 


Nitrogen. . . 


60 


43 


H 2 
CH 4 

C 2^2 
C 2 H 4 


5,085 
35 
65 
15 


97.8 

.7 

1.3 

.2 




• • « QO • • • •• • 


15 


43 


H 2 
CH 4 

C 2" 2 
C 2 H 4 


3,119 

108 

54 

11 


94.7 

3.3 

1.6 

.4 






15 


43 


CH 4 


5 


100 






60 


43 


CH 4 


27 


100 






39 


43 


CH 4 


<5 


NC 




Nitrogen. . . 


30 


43 


H 2 
CH 4 
C 2 H 2 
C 2 H 4 

C 3 H 6 


17,797 

217 

283 

54 

<5 


96.9 
1.2 
1.6 
0.3 







30 


21.5 


CH 4 
C 2 H 2 


5 
<1 


NC 
NC 




Nitrogen. . . 


30 


21.5 


H 2 
CH 4 
C 2 H 2 
C 2 H 4 


7,458 

245 

106 

17 


95.3 

3.1 

1.4 

.2 






30 


21.5 


CH 4 
C 2^2 
C 2 H 4 


7 
5 
3 


46.7 
33.3 
20.0 




Nitrogen. . . 


30 


21.5 


H 2 
CH 4 

C 2^2 
C 2 H 4 


11,186 

580 

1,030 

135 


76.2 
7.9 

14.0 
1.9 


DPR 242 


• • *QO« • » t • i 


30 


43 


H 2 
CO 
CO 2 
CH 4 

2 2 
C 2 H 4 

C 2 H 6 


5,593 
1,300 
580 
565 
500 
413 
13 


79.0 

NC 

NC 

8.0 

7.1 

5.8 

.1 






30 


43 


CO 
CO 2 
CH 4 
C 2^2 
C 2 H 6 


210 

4,700 

9 

7 

9 


NC 

NC 

50.0 

38.9 

.2 




Nitrogen. . . 


30 


21.5 


H 2 
CO 2 
CH 4 
C 2 H 2 
C 2 H 4 


2,034 

350 

130 

217 

98 


82.0 

NC 

5.2 

8.8 

4.0 






30 


21.5 


CO 2 
CH 4 
C 2^2 


1,700 
3 
3 


NC 
50.0 
50.0 



TABLE 2. - Arc-decomposition products — Continued 



Material 


Environment 


Cycles 


Average energy 
per cycle, J 


Decomposition 


products 


Total combus- 




Component ' 


ppm 


tibles , % 




Nitrogen. . . 


30 


43 


H 2 

CO 2 

CH 4 

C 2 H 2 

C2H4 

C 2 H 6 


3,898 

1,700 

576 

609 

315 

9 


72.1 

NC 

10.6 

11.3 

5.8 

.2 






30 


43 


CO 
CO 2 
CH 4 
C 2 H 2 


340 

4,800 

14 

11 


NC 

NC 

56.0 

44.0 




Nitrogen. . . 


30 


21.5 


H 2 
CO 

co 2 

CH 4 
C 2 H 2 
C 2 H 4 
C 2 H 6 


3,390 
480 
300 
217 
250 
109 
12 


85.0 

NC 

NC 

5.5 

6.3 

2.7 

.3 






30 


21.5 


C0 2 
CH 4 

C 2"2 
C 2 H 4 


1,600 

14 
5 
3 


NC 
63.6 
22.7 
13.7 




Nitrogen. . . 


30 


43 


H 2 
CH 4 
C 2 H 2 
C 2 H 4 


1,661 

174 

43 

9 


88.0 

9.2 

2.3 

.5 






30 


43 


CO 2 
CH 4 


1,300 

7 


NC 
100 




Nitrogen. . . 


30 


21.5 


H 2 
CO 

CH4 

2 2 
C 2 H 6 


2,034 

330 

65 

17 

9 


95.7 

NC 

3.1 

.8 

.4 






30 


21.5 


CO 2 
CH4 

C 2 H 2 


1,100 

14 

2 


NC 
87.5 
12.5 




Nitrogen. . . 


30 


43 


H 2 
CO 
CO 2 

CH 4 

C 2 H 2 

C 2 H 4 


5,593 

1,300 

150 

228 

1,011 

217 


79.3 

NC 

NC 

3.2 

14.3 

3.2 






39 


43 


CH 4 

C 2 H 2 

C 2 H 4 


3 
6 
3 


25.0 
50.0 
25.0 




Nitrogen. . . 


30 


21.5 


H 2 

CH 4 

C 2 H 2 

C 2 H 4 


1,186 
33 

87 

27 


89.0 
2.5 
6.5 
2.0 




Air 






CO 2 
CH 4 
C 2 H 2 


2,200 
3 

6 


NC 
33.3 
66.7 



NC Not considered in the calculation of percentage t 
H 2 = hydrogen; CH 4 = methane; C 2 H 2 = acetylene; 
C3H4 = propyne; C3H5 = benzene. 



otal. 

C 2 H 4 = 



ethylene; C3H 6 = propylene; 



10 



In order to routinely test X-P enclo- 
sures for structural performance as spe- 
cified in 30 CFR 18, the apparatus shown 
in figure 2 is required. This setup 
consists primarily of a water reservoir, 
a regulated nitrogen source, intercon- 
necting hardware, and pressure-sensing 
instrumentation. 

The water tank should be at least 50 
gal in volume and be rated for 300-psig 
water service. A regulated high-pressure 
bottle of nitrogen (2,000 psig), or the 
equivalent, is used to provide the input 
pressure to the water tank. The geome- 
try, size, and complexity of the test en- 
closure will dictate how it is connected 
to the pressurization apparatus. 

Once a pressure connection is avail- 
able, the next step is to provide a tight 
seal between the cover and the enclosure 
if one has not been provided by the manu- 
facturer. Two sealing methods have been 
found to be satisfactory for hydrostatic 



testing. The most suitable method for 
the enclosure being tested can be used. 

Me thod 1 (suitable for small, regular 
shaped enclosures). — In this method, a 
continuous gasket is cut from 1/16-in- 
thick reinforced neoprene. The gasket 
should extend outside the bolt circle, 
but no closer than 3/16 in from the outer 
edge of the flange or cover. Clean the 
mating surfaces with acetone or an equiv- 
alent solvent before placing the gasket 
and closing the enclosure. No sealants 
are necessary. Torque bolts to their 
rated load. 

Method 2 (suitable for enclosures of 
all sizes) . — This method uses General 
Electric Silicon Construction Sealant 
1200 series. Clean the surfaces to be 
sealed with acetone or an equivalent 
solvent and apply the sealant in a uni- 
form bead 1/16 to 1/8 in. in diameter. 
A zigzag pattern, as shown in figure 3, 
has been found to work satisfactorily. 



Test cell Control cell 

i l/4-in steel tubing and fittings 



— £>£}— Vent 
valve 



r\ 




Water 
tank 



To water .x. 
supply ~~ ^^ 




Level 
gage 



Vent 
■fcl<j- 4 -conductor 



Test 

item Um 



-O- 

Pressure 



t 



transducer 



"^S: 



T^S, 



Reference 
dial gage 



® 

L 



t&\ — Vent 
valve 



Nitrogen . 
cutoff X* 
valve 



Water cutoff 
valve 

1£] 1 



aU 



Digital voltmeter 



d§ 



i 

i — 



tt 



Power supply 



X\\\\\V 



Regulated 

nitrogen 

bottle 



X\ 



FIGURE 2. - Hydrostatic test apparatus. 



11 



Sealant 




FIGURE 3. - Recommended sealant pattern. 

Some sealant will be extruded from the 
outer edge of the enclosure by this 
method, but there will also be spaces 
for a feeler gage. Use heat lamps to 
raise sealant temperature above 110° F 
(43.3° C) for a faster cure. The lid can 
be applied after the sealant "skims" or 
after it has cured completely. High 
humidity and high heat reduce the cur- 
ing time. A faster cure should occur 
without the cover in place. Torque bolts 
to their rated loads. 

For both methods, prior to connecting 
the enclosures to the pressurization sys- 
tem and bolting the cover, check all sur- 
faces for flatness so that a pretest 
baseline can be established. Deforma- 
tions, if any, due to the hydrostatic 
test can then be determined. Use a steel 
straightedge against every flat surface; 
measure and record any undulations. Mea- 
sure across the width and length of each 
side, and, if necessary, across the two 
diagonals . 

When all of the pretest measurements 
have been recorded, place the enclosure 
on the test stand with the sealing sur- 
face level and at the highest point on 
the enclosure. Connect the enclosre to 



the pressurization system and completely 
fill the enclosure with water. Eliminate 
or minimize air pockets when filling the 
enclosure. Apply the sealant or gasket 
and allow it to cure if necessary. Cur- 
ing time can be accelerated with heat 
1 amp s . 

The enclosure is now ready to be closed 
for pressurization. Top the enclosure 
with water if necessary (water can over- 
flow) , open the vent valve, install cov- 
er, and torque bolts to rated load. Us- 
ing feeler gages, measure and record the 
gap, if any (between the enclosure and 
its cover) on all sides of the enclosure, 
with at least one measurement between 
each pair of bolts. If the extruded 
sealant precludes insertion of the feeler 
gages, it can be easily removed with a 
fine-bladed knife. 

After an enclosure has been connected 
to the water tank, the hydrostatic test 
is conducted as follows: 

1. Open regulator and gradually ap- 
ply pressure to the system in 30-psig 
increments . 

2. After each pressure step, close 
nitrogen cutoff valve and inspect the 
enclosure for water leaks. Very small 
leaks can be tolerated, particularly at 
the higher pressures. 

3. In case of a flowing leak, shut off 
nitrogen cutoff valve, close the water 
cutoff valve, and close the regulator. 
Open vent valve by reference dial gage. 
Reseal the enclosure before starting test 
again. 

4. After the test pressure reaches 150 
psig, close the nitrogen cutoff valve. 

5. Shut off the regulator and open 
vent valve by reference dial gage. 

6. Close water cutoff valve. 

7. Measure and record the gap between 
the enclosure and its cover at the same 
locations surveyed after sealing. 

8. Remove enclosure. 

9. Remeasure and record flatness of 
each side to determine if any permanent 
deformations resulted 
static pressure test. 

The enclosure will 
structural performance 



from the hydro- 



have passed the 
test if (1) no 



deformations larger than 0.04 in/ft are 



12 



measured on any side, and (2) the gap has 
not increased more than 0.002 in relative 
to pretest measurements. 

RUGGEDNESS 

The environment in which X-P enclosures 
must operate underground can be so severe 
that loads produced by internal methane- 
air explosions may not govern the design 
of some components of the enclosure 
(i.e., covers and sides). Other loads 
may be greater. As enclosures evolve, 
the venting of enclosures through X-P 
vents may become permissible. If this 
occurs, the internal pressure loads may 
be quite low and other design criteria, 
such as ruggedness, will control. The 
purpose of this work was to evaluate the 
environment in the mines and to identify 
suitable criteria that an enclosure 
should meet to ensure that it will func- 
tion properly. 

To estimate the forces that can occur 
in typical mining operations (forces that 
can be applied to exposed surfaces of 
X-P enclosures), operating mines were 



examined for evidence of damage , and 
forces that might be produced by differ- 
ent mine accidents were calculated. The 
forces evaluated were — 

° Forces required to produce observed 
damage in a mining machine. 

o Contact forces when a mining machine 
strikes a rib. 

o Impact forces when a shuttle car 
rams a mining machine. 

° Impact forces produced by roof 
falls. 

Figure 4 gives the kinetic energy (KE) 
per unit area at the top of a continous- 
mining machine versus the cumulative per- 
centage of roof falls (1_) . The height 
was taken as 6 ft above the mine floor, 
but no higher than 1 ft below the mine 
roof. For X-P enclosures located on top 
of the mining machine, the height above 
the mine floor was taken as 3 ft. The 
data are bounded by upper and lower 
curves , and values are read from the 
lower bound, which gives the highest KE 
at any percentage level. For example, 
60% of all roof falls should produce 
KE levels equal to or less than 3,200 



100 



o 
a. 

> 

E 

O 
CO 



O 
O 

or 




4 6 8 10 12 14 16 18 

KINETIC ENERGY 3 FT ABOVE MINE FLOOR, I0 3 fflb/ft 2 

FIGURE 4. - Kinetic energy produced by roof falls at top of continuous-mining machine. 



13 



ft-lb/ft 2 . Thus, to protect against 60% 
of all roof fall accidents, X-P enclo- 
sures should be designed for a KE level 
of 3,200 ft-lb/ft 2 . To protect against 
90% of all roof falls, the enclosure 
should be designed for a KE level of 
16,000 ft-lb/ft 2 . 

In summary, the following recommenda- 
tions are made: 

° Test enclosure surfaces exposed to 
the roof at a KE level that will protect 
against 60% of roof falls. 

° Test enclosure surfaces exposed to 
side impacts to an energy level that cor- 
responds to a 60,000-lb shuttle car im- 
pacting at 2 mph. 



° Design protected surfaces to a KE 
level that is 10% of that required for 
side impact. 

These recommendations correspond to the 
following energy levels: 

° Top exposure: Design and test to KE 
= 3,200 ft-lb/ft 2 . 

° Side exposure: Design and test to 
KE = 8,000 ft-lb/ft 2 . 

° Protected areas 
operator's canopy): 
KE = 800 ft-lb/ft 2 . 

Only impact forces produced by roof 
falls are discussed here. Detailed in- 
formation on the other forces are con- 
tained in a Bureau contract report (1). 



(such as under the 
Design and test to 



EFFECTS OF HIGH-VOLTAGE ON EXPLOSION-PROTECTION TECHNIQUES 



The development of more efficient coal 
removal equipment for use in underground 
coal mines has resulted in a demand for 
more electrical power at the working 
face. This could be accomplished in sev- 
eral ways; however, the most cost-effec- 
tive way appears to be the increase of 
the supply voltage. The Bureau is re- 
viewing the effects of increasing the 
high voltage limit (presently limited to 
4,160 V by MSHA) to 15,000 V. One aspect 
of this review is the effect of this new 
limit on explosion protection techniques. 

The Bureau has conducted research on 
the three methods of explosion protec- 
tion: X-P enclosures, pressurization, 
and potting (_5 ) . The three methods pre- 
sent varying degrees of difficulty in 
protecting the mine environments from 
potential ignition. The effects of high 
voltage on conductor isolation, insulator 
selection, and power dissipation must be 
considered when comparing explosion pro- 
tection techniques. 

High-voltage conductor isolation must 
be reliably accomplished within the se- 
lected enclosure based on the available 
energy and the potential for damage to 
enclosure materials during breakdown. 
The theory of operation for the potted 
enclosure assumes inherent conductor iso- 
lation. However, practical application 
of this method requires definition of 
various verification and certification 
tests, the potting compound specifi- 
cation, and the protective enclosure 



specification. The explosion-proof meth- 
od provides conductor isolation by sep- 
aration and insulation. It has been 
demonstrated that separation is not a re- 
liable isolation technique at high volt- 
ages for uninsulated conductors during 
methane combustion. High-voltage conduc- 
tors should be insulated to provide re- 
liable isolation in an X-P enclosure. 
The pressurized enclosure provides con- 
ductor isolation by the separation and 
insulation. However, the inherent lack 
of methane combustion within the enclo- 
sure greatly increases the reliability 
of the isolation method of isolation 
separation. 

A high-voltage insulator must reliably 
isolate the conductor under any condi- 
tions anticipated for the enclosure. The 
selection of an insulation material is 
most critical in the X-P enclosure owing 
to the harsh conditions present during 
methane combustion. These insulators 
must not only survive these combustion 
episodes, but they also must resist the 
effects of corona discharge. The insula- 
tors selected for use in pressurized and 
potted enclosures are not required to 
survive methane combustion due to the 
lack of flammable gas concentrations 
within these enclosures. However, the 
insulators must be resistant to the ef- 
fects of corona discharge. 

The power dissipation of a given 
machine will increase as the supplied 
power is increased. The potted enclosure 



14 



is very sensitive to variations in the 
power dissipation owing to the natural 
thermal insulation of the potting com- 
pound. Small changes in the power dissi- 
pated within this enclosure can produce 
large temperature gradients in the pot- 
ting compound. The resultant tempera- 
ture gradients are dependent on the heat 



capacity, conductivity, and thickness of 
the potting compound. The pressurization 
and X-P enclosures provide air circu- 
lation around equipment by forced circu- 
lation or convection. Thus, these two 
methods are relatively insensitive to 
changes in the machine power dissipation. 



INNOVATIVE X-P DEVICES 



PRESSURE VENT 

The design requirements for X-P enclo- 
sures used in gassy areas of underground 
mines are contained in 30 CFR 18. These 
regulations define an X-P enclosure as 
"an enclosure that complies with the ap- 
plicable design requirements in Subpart B 
of this part and is so constructed that 
it will withstand internal explosions of 
methane-air mixtures: (1) Without damage 
to or excessive distortion of its walls 
or cover(s) and (2) without ignition of 
surrounding methane-air mixtures or dis- 
charge of flame from inside to outside 
the enclosure." The intent of this regu- 
lation is to provide criteria to ensure 
that the explosive energy will be con- 
tained within the enclosure. 

Since internal pressure may exceed 100 
psig, these enclosures are characterized 
by heavy wall construction, tight flange 
gaps, and a multitude of cover bolts. If 
an enclosure could be designed to release 
or vent the heat and pressure in a safe, 
controlled manner, the design criteria 
of 30 CFR 18 could be relaxed to permit 
lighter enclosures and less stringent 
flame path requirements. Lightweight 
construction would facilitate handling of 
large enclosure covers or complete enclo- 
sures by one person. In addition, larger 
allowable flange gaps would negate the 
present effects that corrosion and dust 
entrapment have on attempts to comply 
with 30 CFR 18. 

Under contract H0357107 (7-8), the Bu- 
reau investigated numerous concepts in- 
cluding venting, designed to reduce the 
internal pressure generated in X-P 
enclosures during internal methane-air 
explosions. From this study, it was 
determined that any venting mechanism 
must — 



Quench both methane gas and coal 
dust flame fronts. 

o Be relatively permeable to gas flow 
to minimize vent size. 

° Have self-cleaning characteristics 
to reduce the possibilities of clogging 
during use underground. 

o Have sufficient corrosion and me- 
chanical shock resistance to be compati- 
ble with the mine environment. 

In light of these requirements , an 
open-cell metal foam was judged to offer 
the best combination of mechanical and 
flame-arresting properties. For actual 
hardware designs, Retimet, a stainless 
steel foam from the United Kingdom, was 
chosen; it is an excellent flame arrestor 
and was the only open-cell metal form 
available. For the prototype vent, a 
1/2-in-thick foam slab of Retimet was 
fixed in a metallic frame and mounted in 
the enclosure wall. A cross-sectional 
view is shown in figure 5. The foam was 
protected against mechanical damage by a 
hinged metal cover that swung open when 
internal pressure exceeded 2 psig. Nor- 
mally, this cover was held in place by a 
small permanent magnet. 

To have a significant impact upon en- 
closure design, it was determined that a 
vent must limit internal explosion pres- 
sure to a 12-psig maximum. To determine 
the relationship between vent area size, 
the internal pressure developed, and the 
enclosure volume, a vent area-to-volume 
ratio was established through laboratory 
tests. As illustrated in figure 6, any 
enclosure with a vent area to volume ra- 
tio larger than 4 in 2 /ft 3 will meet the 
12-psig criterion. 

To protect the metal foam from oxida- 
tion damage due to high temperature, a 
series of 20-mesh stainless steel wire 



15 



Vent body 



Vent cover 
retainer 

Swinging- door 
vent cover 

Magnet holds 
cover closed 




Enclosure cover 



Flame path meets 
MSHA requirements 



Steel retaining ring 
vent retainer 



Retimet vent 
material, 
foam stainless 
steel 



Bolt with lock 
washer 



Bolt with lock 
washer 



^jC(j Tapped blind hole 

as required by MSHA 



FIGURE 5. - Pressure vent. 

screens was placed on the inner face of 
the foam and acted as a thermal barrier 
to the flame front. The minimum number 
of screens to be used increases as the 
vent area-to-enclosure volume ratio de- 
creases. The number of screens used in 
the vent design must be more than or 
equal to the number of screens indicated 
in figure 7. 

It was determined that flange-gap tem- 
peratures must not exceed 1,200° F 
(649° C) peak to preclude ignition of a 
surrounding external gassy atmosphere. 
Since one of the goals of utilizing pres- 
sure vents was to permit the safe use of 
larger flange gaps, a series of labora- 
tory tests was conducted to determine 
allowable gaps for various vent area-to- 
enclosure volume ratios. The maximum al- 
lowable gap was found to decrease as the 
vent area-to-enclosure volume decreases 
(see figure 7). 

To use figure 7, the designer deter- 
mines a convenient size for the pressure 
vent assembly or the enclosure. Using 
this, one then can determine the number 
of screens required from the top axis and 
the maximum flange gap spacing from the 



left axis. The enclosure could then be 
designed solely on the basis of rugged- 
ness for the mine environment. 

ELASTOMERIC GROMMET CABLE ENTRY 

Conventional asbestos-packed cable en- 
tries require considerable skill, ex- 
perience, and motivation on the part of 
the mine mechanic to achieve a permis- 
sible installation. Proper cable entry 
is time-consuming. It not only entails 
packing the correct length of asbestos 
rope in the stuffing box but also tight- 
ening the gland nut to the correct depth 
in accordance with 30 CFR 18. 

Under contract H0357107 (7-8), the Bu- 
reau investigated numerous cable entry 
concepts in addressing the deficiencies 
of asbestos entries. A cross section of 
the optimum design is shown in figure 8. 
It incorporates a tapered elastomeric 
grommet compressed against the cable 
jacket by a clamping nut. The advantages 
of this arrangement are as follows: 

° Reduced entry time. 

° Reusability. 

° One size fits many cable diameters. 

° Captive grommet cannot be lost. 

Evaluation of several materials led 
to the selection of polyurethane as the 
grommet material. It is highly resistant 
to wear and abrasion, and impervious to 
oil, grease, and water. Laboratory tests 
of the grommet proved its equivalency to 
conventional asbestos packing in the fol- 
lowing areas: 

° Ability to grip the cable. 

° Gland nut torque required. 

° Distance in contact with cable. 

o Flammability. 

° Explosion-proof integrity. 

Both the pressure vent and the grommet 
cable entry were successfully field 
tested at an active underground coal 
mine. Approval for this demonstration 
was obtained via an experimental permit 
from MSHA. The innovative devices were 
installed on the connection box of a 
Jeffrey 120M continuous-mining machine 
for 8-1/2 months. Subsequent laboratory 
tests revealed no measurable deteriora- 
tion in their condition. 



16 



'to 

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CO 
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20 


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1 


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15 


— 


I 12-psig criterion 




— 


10 
5 




1 l 1 i 1 . 1 . 1 — I 1- r- 


— J— 


1 1 







4 8 12 16 20 24 28 

VENT AREA PER ENCLOSURE VOLUME, in 2 /ft 3 

FIGURE 6. - Pressure buildup in vented enclosures. 



32 



o_ 
< 

CD 

i 

UJ 
CD c 

5" 
< CD 



5 CO 



0.05 



.04 







16 



10 



NUMBER OF SCREENS 
6 3 



T 



No screens over 28in 2 /ft 




Guideline curve 



8 12 16 20 24 28 32 

VENT AREA PER ENCLOSURE VOLUME RATIO, in 2 /ft 3 

FIGURE 7. - Suggested guidelines for number of screens and allowable flange gaps for vents and 
explosionproof electrical enclosures. 



17 



Enclosure wal 



Retaining clip 

Tapered urethane 
grommet 



Thrust 
washer 



Clamping 
nut 




Cable body 
entry 




FIGURE 8. - Trai ling-cable entry assembly. 



SUMMARY 



Research by the Bureau has revealed 
much about the mechanisms of contain- 
ing a methane-air explosion. Finite- 
element analysis and hydrostatic pressure 
tests revealed wide variations in the 
margin of safety of selected X-P enclo- 
sures. Results of research on electrical 
clearances showed that clearances that 
would be safe in ordinary locations are 
insufficient when there is a possibility 
of a methane-air explosion. The use of 
glass and polycarbonate have proved sat- 
is factory for window materials in X-P 
enclosures with certain restrictions. 

when choosing potting materials for 
use in X-P enclosures, attention must 



be given to the possibility of the accu- 
mulation of arc-decomposition products, 
especially when used in high-voltage en- 
closures. The lack, of definition, speci- 
fications, and basic research on most 
potting materials make this method a dif- 
ficult choice. The X-P enclosure and the 
pressurized enclosure can be applied di- 
rectly to high-voltage conditions using 
the present state of the art. Addition- 
ally, research results showed that the 
pressure vent precludes a buildup of high 
pressure caused by an explosion inside a 
permissible enclosure, and the elasto- 
meric grommet simplifies and hastens ca- 
ble entry while minimizing worker error. 



REFERENCES 



1. Cox, P. A., 0. H. Burnside, E. D. 
Esparza, F. D. Lin, and R. E. White. A 
Study of Explosionproof Enclosures (con- 
tract H0377052, Southwest Res. Inst.). 
BuMines OFR 96-83, 1982, 426 pp.; NTIS PB 
83-205450. 



2. Scott, L. W. , and J. G. Doglos. 
Electrical Arcing at High Voltage During 
Methane-Air Explosions Inside Explosion- 
proof Enclosures. BuMines TPR 115, 1982, 
9 PP. 



18 



3. Scott, L. W. Some Design Factors 
for Windows and Lenses Used in Explosion- 
proof Enclosures. BuMines IC 8880, 1982, 
9 pp. 

4. U.S. Code of Federal Regulations. 
Title 30 — Mineral Resources; Chapter I, 
Mine Safety and Health Administration. 
U.S. Department of Labor; Subchapter D — 
Electrical Equipment, Lamps, Methane 
Detectors; Tests for Permissibility; 
Fees; Part 18 — Electrical Motor-Driven 
Mine Equipment and Accessories; July 1, 
1984. 

5. Herrera, W. R. , R. E. White, L. M. 
Adams, and H. S. Silvus. Recommended Ac- 
ceptance Criteria for Potting Materials 
Used in Explosionproof Enclosures in Coal 
Mines (contract J0100041, Southwest Res. 
Inst.). BuMines OFR 168-83, 1982, 100 
pp.; NTIS PB 83-254722. 



6. Linley, L. J., and A. B. Luper. 
Performance Criteria Guideline for Three 
Methods of Electrical Equipment Rated Up 
to 15,000 Volts AC (contract J0318081, 
NASA). BuMines OFR 60-84, 1982, 63 pp.; 
NTIS PB 84-172303. 

7. Gunderman, R. J. Innovations for 
Explosionproof Electrical Enclosures 
(contract H0357107, Dresser Ind., Inc.). 
BuMines OFR 121-81, 1980, 109 pp.; NTIS 
PB 82-104936. 

8. . Evaluation and Acceptance 

Criteria for Innovations in Explosion- 
proof Electrical Enclosures (contract 
H0357107, Dresser Ind., Inc.). Bu- 
Mines OFR 127-83, 1982, 141 pp.; NTIS PB 
233379. 



irU.S. CPO: 1985-505-019/20,111 



IN T.-BU.O F MINES,PGH.,P A. 28121 



Ml 1 



5 



U.S. Department of the Interior 
Bureau of Mines— Prod, and Distr. 
Cochrans Mill Road 
P.O. Box 18070 
Pittsburgh. Pa. 15236 



OFFICIALBUSINESS 
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